The series hybrid electric vehicle designates an electric vehicle configured to drive an electric generator by an engine, to supply electric power from the electric generator to a motor, and to drive wheels by the motor, as disclosed in Japanese Laid Open Patent Application JP-A-Heisei, 4-322105, for example. In a series hybrid electric vehicle, the engine is dedicated to the electric generation, and the motive energy generated by the engine is not mechanically transmitted to the drive wheels, differently from a parallel hybrid electric vehicle
An advantage of the series hybrid electric vehicle lies in that the series hybrid electric vehicle ideally has high energy efficiency. The drive train mechanism in the parallel hybrid electric vehicle is complicated, since both of the engine and the motor are mechanically connected to the drive wheels. A complicated drive train mechanism may cause the loss of the energy. On the contrary, the drive train mechanism of the series hybrid electric vehicle can be simplified. The simplification of the drive train mechanism is effective for minimize the loss, and reducing the weight. Therefore, the series hybrid electric vehicle is excellent in the ideal energy efficiency.
In order to make good use of the foregoing advantage, the series hybrid electric vehicle is required to ultimately improve the energy efficiency. The use of a mechanism that decreases the energy efficiency is not preferred, because it loses the advantage of the series hybrid electric vehicle.
The first point in the improvement of the energy efficiency of the series hybrid electric vehicle is the loss in the electric generation system. The electric generation system is typically provided with an electric generator and an inverter for converting an alternating voltage generated by the electric generator into a direct voltage. It is difficult to avoid some loss, since the inverter is composed of switching transistors. Typically, the loss of an inverter is about 5%. However, the loss of 5% in the inverter is of importance in the series hybrid electric vehicle, where the energy efficiency is emphasized.
The second point is improvement in the efficiency of the drive system. In the series hybrid electric vehicle where the high energy efficiency is requested, the losses of a transmission mechanism and a differential mechanism are also unignorable. The series hybrid electric vehicle is required to employ a drive system that avoids the losses in the transmission mechanism and the differential mechanism.
Therefore, there is a need for providing a technique for improving the energy efficiency of a series hybrid electric vehicle, more specifically, for further improving the efficiencies of an electric generation system and a drive system within a series hybrid electric vehicle.